Tuning system



INCREASING,-

Feb. 23, 1943. I R. DE COLA 2,312,211

TUNING SYSTEM Filed Sept. 3, 1940 Patented Feb. 23, 1943 TUNING SYSTEMRinaldo De Cola, Chicago, mont Radio Corporation,

ration of Illinois 111., assignor to Bel- Chicago, 11]., a corpo-Application September 3, 1940, Serial No. 355,120

1 Claim.

My invention relates in general to a tuning system for a high frequencycircuit to maintain a relatively constant wide-band over a relativelylarge frequency range, and more in particular to such a system for atelevision essentially constant band widths, a flat response, and highgain over the entiretuning range of the receiver.

Automatic, or push-button tuning as it is variously called, isparticularly useful in television because of the relatively few, andsometimes spaced apart channel for broadcasting, spread over aconsiderable range of frequencies. With the ordinary manual tuning oversuch a relatively large tuning range, considerable time is required tocover this range, an unskilled operator often has diificulty inobtaining optimum tuning, and in addition, manual tuning necessitatesremembering the various frequencies for each station. As is well known,an automatic tuning system substantially eliminates all of thesedifficulties in selecting the desired station and this has beenrecognized in television receivers. However, prior to the presentinvention, television tuning has generally been accomplished by means ofindividual pretuned circuits for each frequency, with switchingmechanism to switch or tune from one pretuned circuit to another. Thesesystems are bulky-and costly, and are inefficient due to lossesincidental to the switching mechanism and the associated pretunedcircuits. Leads between the switch and the first vacuum tube in thereceiver circuit, and in the switch mechanism itself, all combine to addconsiderable stray capacitance and inductance to the tuning system. Thisreduces' gain, and results in extremely unequal signal response atvarious frequencies over the entire tuning range. In fact, because ofstray capacitance and inductance resulting from the above-notedconditions in the prior systems, it is diflicult to obtain completelyeffective tuning under any circumstances, at the higher frequencies.

More particularly, in television preselector or tuning systems it isdesirable to build up a maximum circuit impedance at the terminals ofthe first vacuum tube in the receiver circuit. For example, in ahalf-wave dipole, having a characteristic resistance of approximately 77ohms, it is desirable that the matching transformer working into thefirst vacuum tube amplifier has as high a step-up ratio as possible.This is desirable because the resulting gain from the antenna to thegrid of the first tube will be directly determined by the square root ofthe ratio of the grid circuit impedance to the antenna impedance, and ofcourse, the higher this ratio becom s, the higher the gain becomes.However, the difliculties described with reference to the prior systemsdefinitely diminish this circuit impedance below the receiver to providedesirable maximum, and hence reduce the gain proportionately.

An object of my invention is to provide an improved tuning system forhigh frequency signal circuits.

A further object is to provide an improved tuning system for televisioncircuits.

It is also an object of my invention to provide efficient continuoustuning in a single circuit to predetermined frequencies over the entiretuning range of a television receiver, in contrast to tuning over suchrange through a plurality of pretuned circuits which require extensiveadjustment in Order to change the pretuned frequency setting.

Another object of my invention is to provide a tuning or preselectorsystem for a television circuit in which stray capacitances andinductances are reduced to a minimum, to in turn provide a maximumcircuit impedance and high gain.

Another object is to provide a radio frequency tuning system whichmaintains a substantially uniform wide band at each tuning frequencyover an entire tuning range.

A still further object of my invention is to provide a tuning system fora television circuit effecting a flat response at each signal frequencyover the entire tuning range of the circuit, and hence high fidelity.

One of the features of my invention is the provision of a tuning systemand circuit for a signal receiving circuit which can be compressed intoa relatively small space, with physical components of such type and sizethat they are readily adapted to various mounting positions so that theymay all be placed relatively close to the tube to which they areelectrically connected, thus reducing the lead lengths to a minimum andconsequently minimizing stray capacitances and inductances in the leadsin the system, to thereby provide maximum circuit impedance at the firsttube for that particular circuit.

Another feature of my invention is the provision of variable inductancesfor changing frequency in a television receiver circuit, and couplingmeans for the inductances which provides a substantially uniform wideband over the tuning range of the receiver.

A still further feature of my invention is the provision of apermeability tuning system for television receivers for improving thesensitivity and the selectivity of the receiver over the entire tuningrange thereof.

Other objects, features, and advantages of the invention will beapparent from the following description taken with the drawing in which:

Fig. 1 is a circuit diagram of the tuning portion circuit embodying thein of a signal receiving vention;

Fig. 2 is a graph illustrating certain of the of the circuit of Fig. 1;

operating characteristics tion of my invention Fig. 3 is a fragmentaryplan view in actual size of a chassis showing the physical components ofthe tuning circuit including tube sockets for the.

tenna, and the secondary circuit thereof connected to the first tube inthe receiver circuit and being tuned substantially by the capacity ofsuch tube. In addition to the inductive coupling, capacitance in theform of a condenser is inserted between the two circuits to couple thesame capacitively, with such capacitive coupling providing asubstantially constant band width over the entire tuning range of thefrequency. The physical components of the variable inductances fortuning these coupled circuits are such that they may be convenientlygrouped around the tube to which the coupling system is connected tothus reduce the physical space required for installation, and reduce thelead lengths between the various components of the system so as toreduce undesirable stray inductance and capacitance to a minimum.

Referring to Fig. 1, the reference character In denotes a half-wavedoublet antenna system including a transmission line comprising atwisted pair of appropriate impedance for properly matching the antennato the input circuit of the receiver. The transmission line of theantenna system is connected to the input at H and i2 and is properlyadjusted with reference to the input circuit by means of capacitors l3,I4, I6, and IT. This input circuit including the plurality of capacitorsis commonly known as a balanced input system and operates verysatisfactorily with a doublet antenna system. However, the operais notlimited to this particular input system alone, and other suitableantenna terminations may be employed. Generally, capacitors I3 and I1are smaller in value than capacitors l4 and I6. Either one of thecapacitors l3 and I! can be made adjustable to provide a tuning meansfor the primary circuit of the preselector system of my invention.

Included in the primary circuit of the tuning, coupling or signaltranslating system is a variable inductance unit [8 which is preferablyof the slidable iron-core ity tuned. Inductively coupled to the variableinductance I8 is a similar variable inductance unit IS in the secondarycircuit, and capacitively coupled to inductance I9 is an iron-corevariable inductance unit 20. The latter inductance is in the oscillatortube 1. The three permeability tuned variable inductance-units l8, l9and 20 are adjusted simultaneously by a coupling bar 2|, operativelyconnected with the slidable iron cores therein as shown in Fig. 3, totune a receiver circuit over the entire tuning range or frequencyinterval thereof. The bar 2|, in turn is suitably connected to asuitable push button tuning device (not shown) of either the manuallyoperated type or electrically operated type, with a plurality ofselector units in the device acting to selectively move the cores of-thecoil units to any one of a plurality of predetermined station settings.The bar 2| may likewise beconnected into a manually operated type andhence permeabilcircuit as is evident from Fig.

ating range for structure for moving the cores over the entire tuningrange of the receiver, as might be indicated on an appropriate dialmechanism. With the inductance coil units l8 and I9 permeability tuned,the coupling factor M varies with the frequency to which the circuitstherefor are tuned. At the lowest frequency in the operthe circuit, whenthe cores are completely within the coils for the units, the couplingfactor M is a maximum. However, at the highest frequency in theoperating range, when the cores are practically out of the field of thecoils, the coupling factor M between the two units is considerablyreduced. This relationship is shown in an approximate manner in the fullline curve in Fig. 2 with zero (0) representing the low frequency end ofthe range, and infinity representing the high end. The general shape andslope of the curve is determined by the proximity of the coils, with theslope being greatest When the coils are well removed or displaced fromone another as can be understood from a consideration of Fig. 3, and theslope is a minimum when the coils are spaced closely together. T Theseconditions are generally known and are conditions which the presentinvention overcomes.

This inductive coupling between the units i8 and I9, if used alonewould, when properly adjusted for the low frequency stations in theoperating range, provide too narrow a band-pass at the higherfrequencies. In order to provide an essentially constant band-width overthe entire tuning range, a small capacitance 23 is introduced betweenthe coils in the units l8 and i9 and in series connection with each asindicated in Fig. 1. This may be a small fixed condenser which acts uponthe band width in a manner complementary to that provided by the mutualinductive coupling between the coils i8 and I9.

With the mutual coupling factor M, and the value of capacitance 23properly adjusted, essentially constant band-width is provided over theentire tuning range of the system as shown by the straight line dottedcurve of Fig. 2. This is in contrast to the variable band-width asillustrated in full line in Fig. 2, and as previously described. Sincethe eflects of the capacitance 23 (dotted line curve in Fig. 2) and themutual coupling factor M are additive, the mutual inductive couplingbetween the coils must be so phased as to have a negative sign.

In arriving at the most desirable values for M and the capacitance 23,the coil units i8 and I! are positioned on a chassis 24 with referenceto one another, as shown in Fig. 3, so that the mutual inductivecoupling M between them is sufiicient for a desired band-width at thelow frequency end of the tuning range. With this coupling satisfied, thecapacity.23 is introduced and provided in such a value that the samedesired band-width is obtained at the high frequency end of the tuningrange. With coupling factor M satisfactory at the low frequency end, andthe capacity coupling through the condenser 23 added to the inductivecoupling M providing a satisfactory band-width at the high frequencyend, it is known that the band width is substantially uniform over theentire tuning range.

In order to have the most eflicient tuning over the entire operatingrange of the receiver, it is important that the impedance ratio betweenthe antenna terminals ii and i2 and the grid G of the first tube T inthe tuning circuit be at a maximum. The tube T which is the first tubein the preselector system, or tuning circuit, will have differentcharacteristics depending upon the particular receiver circuit employed,and whether it be for radio or television. For purposes of illustrationfor the present invention, the receiver circuit may be considered to bea superhet'erodyne, and the tube T a mixer tube. It may also bevariously known as a converter tube. The socket TS for this tube mountedon the chassis 24 is shown in Fig. 3. An oscillator tube T is connectedto the inductance coil unit 20 and is considered to be in the tuningsystem for the receiver. The socket for this tube on the chassis 24 isindicated by the reference character TS' in Fig. 3.

As previously mentioned, in the prior preselector or tuning systems, abulky switching mechanism for pretuned circuits prevented a compactflexible assembly for the physical elements of the tuning system.Consequently, the leads to' the tube or tubes in the circuit and to thevarious elements therein, as well as the leads within the switchingmechanism itself, were all generally so long as to introduce straycapacitance and inductance, and reduce the impedance of the circuit, andconsequently the gain thereof.

As is apparent from the bottom 'planrview of the chassis 24, in actualsize, the inductance coil units I8, I9, and 2| are grouped closelyaround the base of the tube sockets TS, andT'S'. The coil units as wellas the other physical components, which will be described in detail,adapt themselves to a mounting that makes possible short wire leadsbetween the sockets and these various elements. As a result theundesirable stray inductance and capacitance is reduced to a minimum andthe impedance to the grid of the first tube is a maximum therebyproviding the maximum gain in the tuning circuit.

A typical example of an installation of my invention includes condensersI3 and I I of 42.4 micromicro-farads each, and condensers I4 and I6 ofIOO-micromicro-farads each. The inductances I8, I9 and 20, are all .21microhenry each, and the condenser 23 is 2.7 micromicro-farads. Thee'ifective capacitance of tube T, represented by the character CT is 15micromicro-farads. The spacing between inductance units I8 and I9,determining the mutual inductance M in this example, is approximately/16 inch, as can be determined from the full scale drawing of Fig. 3.

As to the remaining elements illustrated in Fig. 3, these include asmall adjustable capacitor 25 which allows the secondary circuitincluding the coil unit I9 to be properly tuned. A condenser, orcapacitor 26 permits the adjustment of the oscillator frequency of thetube T. A fixed condenser 21 is incidental to the oscillator circuititself. Reference characters 28 and 29 indicate the grid condenser, andgrid leak respectively for the tube T. A radio frequency choke 3I isconnected to the plate of the tube T to prevent short circuiting of thecapacitor 21 at radio. frequency. The condenser 32 acts to impress someof the oscillator radio frequency voltage upon the control grid of thetube T in order to provide proper mixing of the signal voltage with theoscillator voltage to produce the intermediate frequency when thesuperheterodyne receiver is used. The condenser 33, and resistor 34,provide filtering for the application of AVG voltage to the tube T. Theresistor 36 acts to stabilize the input impedance of the tube T. Acondenser 31, and resistance 38 provide the necessary filtering andresistance for the application of bias voltage to the tube T.

The condenser symbol indicated by the reference character CT issymbolical of the capacitance of the vacuum tube T itself, together withthe incidental capacitance of the tube socket and associated elements onthe socket itself. With the undesirable capacitance reduced in thetiming circuit as the result of the shortened leads and more efficienttransmission of energy, as previously explained, the impedance in thegrid circuit is increased to a maximum. Where the prior art systems, aspreviously described, required loading in the tuning system in order toprovide the proper matching therein, in the present system the firsttube T provides all of such necessary loading to bring the impedance ofthe circuit up to maximum. Loading as used herein means theadding ofresistance to the circuit in order to match the impedances of the twocoupled circuits including the coil unit I8, and the coil unit I9,respectively. In this permeability tuned circuit, therefore, with all ofthe advantages previously described, tube T furnishes all of therequired loading and the gain is a maximum. The capacity of this tube isan irreducible minimum and may vary as the commercial practice improvesthe construction thereof. As a result, a fiat response curve with highgain is provided from the two coupled circuits. Of course, in the priorswitching mechanism for tuning a receiver circuit, resistance can beadded to obtain the necessary loading so that a flat response curveresults, but the gain is so materially reduced that the overallefficiency of the prior system is much lower.

From the preceding description, and from the drawing, it is apparent,therefore, that my invention provides a highly efficient wide-bandcoupling and tuning system in compact flexible form. With the twomutually coupled variable inductance circuits, and complementarycapacitance coupling as well, high gain operation and uniform band-widthis obtained over the entire tuning range of a receiver. The iron corevariable inductances lend themselves readily to connection with pushbutton, or automatic tuning devices which can be set very readily to anydesired frequency, or coupled to manually operated tuning structure sothat tuning of any type can be readily accomplished over ing range ofthe receiver.

Although my invention has been illustrated and described in itspreferred embodiment, it is understood that the invention is not limitedthereby, but is limited only by the scope of the appended claim.

I claim:

A wide band coupling circuit for coupling a source to a vacuum tube loadand provide a substantially constant band width over an entire tuningrange of a receiver including in combination a transformer having a pairof inductively and capacitively coupled inductance tuned inductanceunits and a vacuum tube unit electrically connected to the secondary ofsaid transformer at the high potential end, said tube unit including asocket and a tube, conductor means for electrically connecting all ofsaid units, with said inductance units arranged relative to one anotherand relative to the socket so that the conductor means are short toreduce stray reactances, and with a maximum impedance being provided atthe input electrode of said tube.

RINALDO DE COLA.

the entire tun-

